Fix atomic entity writes and scoped git identity

[yusufm]

Summary

• make entity and events writes atomic via temp-file replacement to avoid truncation/corruption on failed writes
• scope request git identity through git subprocess env overrides instead of mutating process-global os.environ
• preserve text_size_text on sticker batch error renders and add focused regression coverage

Validation

• pytest -q tests/test_git_workflow.py tests/test_web_identity_headers.py tests/test_entities_core_behavior.py

Notes

• the CLI repo-wide locking concern remains broader than inventory post alone and should be handled as a separate follow-up across all mutating CLI paths